In the U.S., 50% of patients who undergo "curative" resection for colorectal cancer suffer recurrent disease. In part, this reflects the absence of techniques to detect occult micrometastases prior to clinically evident recurrence. Guanylyl cyclase C (GC-C) is specifically expressed by normal mucosal and colorectal cancer cells, but not by extra-gastrointestinal tissues and tumors. GC-C appears to be a sensitive and specific marker that can be employed to detect colorectal cancer cells in extra- intestinal sites. Preliminary studies demonstrated that GC-C RT-PCR could detect metastatic colorectal cancer cells in blood from all (34) patients examined with metastatic colorectal cancer. In addition, GC-C mRNA expression measured by RT-PCR detected occult micrometastases in lymph nodes that were free of disease by histopathology. Patients with lymph nodes positive for GC-C were at greater risk for cancer-related mortality compared to patients with lymph nodes that did not express GC-C These observations suggest that for patients undergoing post-operative surveillance for colorectal cancer, GGC RT-PCR may be useful to detect micrometastases and recurrent disease earlier than other methods. This application will translate basic observations from our laboratory into new diagnostic tools for the management of colorectal cancer. In Specific Aim 1, GC-C expression will be examined in blood from control patients who do not have GI mafignancies, with ages ranging from 40 yo to 90 yo, to establish the baseline value for GC-C RT-PCR in blood in the broad population of individuals at risk to develop colorectal cancer. In Specific Aim 2, the relationship between GC-C RT-PCR analysis in blood and metastatic colorectal cancer will be defined. It is expected that GGC RT-PCR will be positive more often in the blood of patients with metastatic colorectal cancer compared to patients without metastatic disease. In Specific Aim 3, the temporal relationship between clinically evident recurrence and a positive GC-C RT-PCR will be compared to that of serum carcinoembryonic antigen (CEA) in serial blood samples collected prospectively from colorectal cancer patients undergoing post-operative surveillance. The prognostic value of GC-C RT-PCR will be compared to serum CEA levels in patients who recur during this study. It is anticipated that GGC RT-PCR will be positive earlier and more frequently than CEA in the blood of patients who recur. The studies proposed will translate preliminary observations concerning the specificity of GC-C expression in human tissues and blood into clinical data which define the diagnostic utility of this novel marker for managing colorectal cancer patients during post-operative surveillance. These studies will form the foundation for future trials utilizing GC-C to identify patients at risk for recurrence who might benefit from therapeutic intervention.